The invention relates to a piezoelectric actuator for media flowing therearound, as disclosed for example by the generically determinative German application DE 198 18 068 A1, and uses of the same.
The generically determinative German application DE 198 18 068 A1 discloses a piezoelectric actuator for media flowing therearound which comprises a piezo stack which is arranged within a deformable isolating material so as to be in direct contact therewith. The isolating material is enclosed by an actuator housing. The actuator housing is formed by a housing shell, which is connected at one end to a dimensionally stable actuator top and at the other end to a dimensionally stable actuator bottom. The actuator top and the actuator bottom are connected to the active main surfaces of the piezo stack. The electrical connecting lines of the piezo stack are led through the actuator top. Furthermore, it is conceivable to lead the actuator connections of the piezo stack up to the end plates (actuator top or actuator bottom) of the actuator and to use one or both end plates as electrical contact surfaces.
Since the electrically insulating isolating material is formed from an elastic plastic, for example silicone, which bears directly against the outer surface of the piezo stack, it must follow the very rapid movement of the piezo stack in use. As a result, there is the risk of cracks forming in the isolating material and the isolating material becoming detached, so that the medium flowing around can get into the piezo stack in a destructive way.
The object of the invention is to increase the service life of such actuators for media flowing therearound.
The object is achieved by forming the housing shell from a limp and/or elastic material, in order that the hermetic separating layer is moved away from the highly active surface of the piezo stack. This reduces the effect of wear between the piezo stack, comprising a piezoelectric ceramic, and the isolating material. This is achieved furthermore by simple structural operations, so that it is possible to dispense with complex sealing arrangements.
An actuator according to the invention permits a free change in length of the actuator, or of the piezo stack, with the effect of at least reducing the probability of the piezo stack being wetted by a particularly aggressive medium flowing therearound, preferably a pressurized fuel.
The housing shell is disposed at a distance from the piezo stack at all points. Furthermore, the length of the housing shell, measured along the surface line, corresponds at least to the maximum extent of the actuator and/or the housing shell can at least be stretched accordingly, in order that the extent of the piezo stack can be accepted by the housing shell.
Since the isolating material is preferably formed largely by an incompressible medium, its dimensional change, for example the formation of a constriction in the extent of the piezo stack, can be taken into account in the dimensioning of the length of the housing shell, in particular in the direction of the extent of the piezo stack.
Furthermore, an electrically insulating fluid, in particular a liquid, and/or a gel is introduced between the housing shell and the piezo stack. The isolating material, in particular a silicone oil, at least largely fills the inside volume of the actuator housing. In this way, the inside volume of the actuator housing is at least largely free from a compressible gas.
In the case of the claimed construction, only the fluidic isolating material has contact with the piezo stack. Therefore, any shearing forces that may occur are small, thereby increasing the service life of the actuator. Against this background, it is also an advantage that, if the isolating material has good to high thermal conductivity, any frictional heat occurring between the piezo stack and the material is removed.
The same applies to the removal of heat from the piezo stack, for which reason the thermal conductivity of the isolating material is preferably equal to or greater than that of the material of the piezo stack.
The same advantage applies to the thermal conductivity of the two end plates (actuator top and actuator bottom), whereby the dissipation of the heat occurring in active operation to the medium flowing around is facilitated and/or improved.
Since the media flowing around the actuator, in particular fuel for the operation of an internal combustion engine, may well have a chemically or in some other way aggressive character, material which is at least largely resistant to these expected stresses is expediently chosen as the material of the housing shell.
In a favorable way, the viscosity of the isolating medium corresponds approximately to that of the medium flowing around, since the loading of the material of the housing shell by the medium flowing therearound is further reduced as a result.
In particular when using actuators of which the end plate(s) is/are used as electrical contacts, the material of the housing shell is appropriately an electrically insulating material.
Depending on the application, for example in hydraulics, it is favorable to form the actuator top with a different cross-sectional area than the actuator bottom. In a further refinement, the actuator top and/or the actuator bottom has a cross-sectional area which is adapted to the respective conditions in use.
A preferred use of actuators according to the invention is in and/or as an injection valve, in particular of an internal combustion engine, preferably in a gasoline or diesel engine. Furthermore, such an actuator may also be used for a proportional valve and/or for a sonotrode.